Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein’s predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultra-dense objects.

A massive and rare explosion on the surface of this neutron star above
-- pouring out more energy in three hours than the Sun does in 100
years -- illuminated the region and allowed the scientists to spy on
details never before revealed. They could see details as fine as the
neutron star's accretion disk, a ring of gas swirling around and
flowing onto the neutron star, as the disk buckled from the explosion
and then slowly recovered its original form after approximately 1,000
seconds. The swirling flow of gas hovering just a few miles from the
surface of a neutron star, itself a sphere only about ten miles across.

All of this was occurring 25,000 light years from Earth, captured
second-by-second in movie-like fashion through a process called
spectroscopy with NASA's Rossi X-ray Timing Explorer.

Dr. David Ballantyne of Canadian Institute of Theoretical Astronomy at the University of Toronto and Dr. Tod
Strohmayer of NASA's Goddard Space Flight Center present this result in
the current issue of Astrophysical Journal Letters. The observation
provides new insight into the flow of a neutron star's (and perhaps a
black hole's) accretion disk, which usually appears far too minute to
resolve with even the most powerful telescopes.

"This is the first time we have been able to watch the inner regions of
an accretion disk, in this case literally a few miles from the neutron
star's surface, change its structure in real-time," said Ballantyne.
"Accretion disks are known to flow around many objects in the Universe,
from newly forming stars to the giant black holes in distant quasars.
Details of how such a disk flows could only be inferred up to now."

Previous X-ray observatories detected iron lines around neutron
stars, but they lacked the sensitivity to measure the shapes of the
lines in detail. Thanks to XMM-Newton’s large mirrors, NASA astronomers found that the iron line is broadened asymmetrically by
the gas’s extreme velocity, which smears and distorts the line because
of the Doppler effect and beaming effects predicted by Einstein’s
special theory of relativity. The warping of space-time by the neutron
star’s powerful gravity, an effect of Einstein’s general theory of
relativity, shifts the neutron star’s iron line to longer wavelengths.

"We've seen these asymmetric lines from many black holes, but this
is the first confirmation that neutron stars can produce them as well,"
Strohmayer added in a recent analysis. "It shows that the way neutron stars accrete matter is not very
different from that of black holes, and it gives us a new tool to probe
Einstein’s theory."

Neutron stars contain the most dense observable matter in the
universe. They cram more than a sun’s worth of material into a
city-sized sphere, meaning a few cups of neutron-star stuff would
outweigh Mount Everest. Astronomers use these collapsed stars as
natural laboratories to study how tightly matter can be crammed under
the most extreme pressures that nature can offer.

"This is fundamental physics," says Sudip Bhattacharyya of NASA’s
Goddard Space Flight Center in Greenbelt, Md. and the University of
Maryland, College Park. "There could be exotic kinds of particles or
states of matter, such as quark matter, in the centers of neutron
stars, but it’s impossible to create them in the lab. The only way to
find out is to understand neutron stars."

To address this mystery, scientists must accurately and precisely
measure the diameters and masses of neutron stars. In two concurrent
studies, one with the European Space Agency’s XMM-Newton X-ray
Observatory and the other with the Japanese/NASA Suzaku X-ray
observatory, astronomers have taken a big step forward.

The
XMM-Newton paper appeared in the August 1 Astrophysical Journal
Letters. The Suzaku paper has been submitted for publication in the
same journal.